Heat-convertible products obtained by reacting amino- or imino-group-containing compounds, e.g., ureas, amides, aminotriazines, and the like, with aldehydes, e.g., formaldehyde, benzaldehyde, etc. have been known for a number of years. Resins obtained by curing such condensation products, e.g., under the influence of heat, possess an excellent combination of physical properties and are widely used in glues, in molding compounds, as finishes for paper and textiles and as surface coatings. The convertible resins can be used per se or they can be further modified before curing, e.g., by alkylation with alcohol, e.g., methanol or butanol, to provide for solubility and compatibility and/or by admixture with other materials capable of co-reacting therewith, such as poly-functional compounds containing hydroxyl groups and carboxyl groups, e.g., glycols, alkyd resins, polyester resins, and the like. This invention broadly is concerned with amino resins which are suitable for all conventional purposes. However, in its most preferred aspects, it is concerned with soluble forms or liquid forms of such amino resin products, which are well known to be superior as coatings for metals, and coatings or impregnants for cloth, paper, and the like. Such convertible resins commonly comprise urea- or melamine-aldehyde condensates or reaction products thereof with alcohols, e.g., methylol ureas, methylol melamines, and alkylated, e.g., methylated and butylated derivatives thereof either alone or in a suitable solvent therefor. These specific amino resins are applied by coating onto three-dimensional substrates, e.g., metal, glass, wood, plastics, such as appliance bodies, plastic windows, and the like, and then curing under the influence of heat. The mechanism of cure contemplated is by condensation and cross-linking to split out H.sub.2 O or ROH or HCHO, etc., and curing can be effected without a catalyst if long enough heating times--of the order of hours and days--are provided. However, for immediate curing, or for curing at more moderate temperatures, an acid is often added to function as a cross-linking catalyst. Among the acidic catalysts that have been used in the past with amino resins can be mentioned boric acid, phosphoric acids, acid sulfates, sulfonic and sulfonyl halides, hydrochlorides, ammonium phosphates and polyphosphates, acid salts of hexamethylene tetramine, phthalic acid, oxalic acid, and the like.
In U.S. Pat. No. 3,979,478, assigned to the assignee of this application, it was disclosed that high molecular weight polyalkylaromatic polysulfonic acids, such as dinonylnaphthalene disulfonic acids, are superior catalysts for curing amino resin systems. The patent teaches that these catalysts cure resin systems in short periods of time and produce a resin product having superior physical properties. It was also disclosed that these polyalkylaromatic polysulfonic acids can be stored in the form of thermally-decomposable adducts to provide greater shelf life for the catalyst material. In U.S. Pat. No. 3,474,054, the patentee teaches that amine salts (preferably tertiary amine salts) of aromatic sulfonic acids (e.g., the pyridine salt of para-toluene sulfonic acid) can be utilized to cure amino resin coating compositions. And in U.S. Pat. No. 3,293,324, it is disclosed that the 2-dimethylamino-2-methyl-1-propanol salt of para-toluene sulfonic acid can also be utilized to cure thermosetting aminoplast resins.
It has now been unexpectedly discovered that while certain adducts may be useful for curing amino resin compositions, there are particular adducts which impart outstanding resistance properties to the cured resinous product. Applicant has surprisingly found that aromatic sulfonic acids in association with oxa-azacyclopentanes, such as 4,4-dimethyl-1-oxa-3-aza-cyclopentane, are outstanding latent catalysts for curing amino resin compositions. Moreover, it has been found that these compositions in addition to having superior properties, particularly water resistance properties, in comparison to compositions cured with unneutralized acids, are also superior to other amine adducts of the acids. In addition, the compositions of the invention have exceptional package stability so that they can be stored ready for use for relatively long periods of time without significantly detracting from the usefulness of the compositions.
While it is not clearly understood at this time why the above-described advantageous results are obtained, it is believed that these results may be attributable to a unique combination of the high volatility of the amine component and a low dissociation constant incident to the oxa-azacyclopentane adducts. This combination, in addition to other factors, may provide for a particularly effective association of the adduct with the resin during the curing process to provide for fast curing times and a resin product having superior properties.